Massive blooms of pelagic Sargassum algae have caused serious problems to coastal communities and ecosystems throughout the tropical Atlantic, Caribbean Sea, and Gulf of Mexico since 2011. Efforts to monitor and predict these occurrences are challenging owing to the vast area impacted and the complexities associated with the proliferation and movement of Sargassum. Sargassum Inundation Reports (SIRs) were first produced in 2019 to estimate the potential risk to coastlines throughout the Intra-American Sea at weekly intervals at 10 km resolution. SIRs use satellite-based data products to estimate beaching risk from the amount of offshore Sargassum (quantified by a Floating Algal density index). Here we examine whether including wind metrics improves the correspondence between the offshore Floating Algal density index and observations of Sargassum along the coastline. For coastal observations, we quantified the percent coverage of Sargassum in photos obtained from the citizen science project “Sargassum Watch” that collects time-stamped, georeferenced photos at beaches throughout the region. Region-wide analyses indicate that including shoreward wind velocity with SIR risk indices greatly improves the correspondence with coastal observations of Sargassum beaching compared to SIR risk indices alone. Site-specific analyses of photos from southeast Florida, USA, and data from a continuous video monitoring study at Puerto Morelos, Mexico, suggest potential uncertainties in the suite of factors controlling Sargassum beaching. Nonetheless, the inclusion of wind velocity in the SIR algorithm appears to be a promising avenue for improving regional risk indices.

Mean sea-level rise (MSLR) will induce shoreline recession, increasing the stress on coastal systems of high socio-economic and environmental values. Localized mega-nourishments are meant to alleviate erosion problems by diffusing alongshore over decades and thus feeding adjacent beaches. The 21-st century morphological evolution of the Delfland coast, where the Sand Engine mega-nourishment was built in 2011, was simulated with the Q2Dmorfo model to assess the Sand Engine capacity to protect the area against the effects of MSLR. The calibrated and validated model was forced with historical wave and sea-level data and MSLR projections until 2100 corresponding to different Representative Concentration Pathways (RCP2.6, RCP4.5 and RCP8.5). Results show that the Sand Engine diffusive trend will continue in forthcoming decades, with the feeding effect to adjacent beaches being less noticeable from 2050 onward. Superimposed to this alongshore diffusion, MSLR causes the shoreline to recede because of both passive-flooding and a net offshore sediment transport produced by wave reshaping and gravity. The existing feeding asymmetry enforces more sediment transport to the NE than to the SW, causing the former to remain stable whilst the SW shoreline retreats significantly, especially from 2050 onward. Sediment from the Sand Engine does not reach the beaches located more than 6 km to the SW, with a strong shoreline and profile recession in that area, as well as dune erosion. The uncertainties in the results are dominated by those related to the free model parameters up to 2050 whilst uncertainties in MSLR projections prevail from 2050 to 2100.

Coastal erosion is critical in many locations along the northern Yucatan Peninsula. The area is characterized by a micro-tidal regime and low-energy wave conditions, with a high-incidence angle with respect to the shoreline. Port and harbor infrastructure for fisheries, commercial, and tourist activities has promoted the growth of coastal communities settled on barrier islands. However, the human settlements have degraded the coastal ecosystems and interrupted the littoral transport. Due to coastal development in the region, the land use of the remaining pristine coastal areas is expected to change in forthcoming years. Thus, understanding coastal changes occurring along the northern Yucatan Peninsula is fundamental for improving coastal planning. We employed open access remote sensing data sets and reanalysis information to investigate shoreline changes at different spatial and temporal scales. Shoreline position was obtained along a 150-km stretch of coast from satellite imagery using CoastSat. Firstly, reanalysis and satellite-derived information were validated with in situ measurements in the vicinity of coastal structures. A satisfactory agreement was found for characterizing the forcing conditions (waves and sea level) and shoreline evolution at different temporal scales. A dominant direction of alongshore sediment transport (50,000–80,000 m³/year) make the shoreline highly sensitive to any nearshore disturbance. We found that coastal erosion occurred in 50% of the analyzed transects, whereas beach accretion occurred in only 30%, suggesting net beach losses. Erosive trends are strongly correlated with the presence of coastal structures. The 6-km long Progreso pier induced significant beach erosion along O(10) km, while sheltered harbors induced downdrift erosion along O(1) km. Detached breakwaters and groins have an overall negative impact on downdrift areas (O(100) m). On the other hand, significant erosion was also observed in pristine areas located downdrift of a coastal lagoon due to the sediment impoundment associated with the growth of a sand spit. Moreover, shoreline sand waves drive 40-m shoreline oscillations and propagate (alongshore) at a rate of 300 m/year. The generation of sand waves seems to be related to both natural and anthropogenic perturbations, in combination with the high-incidence wave angle. Their propagation plays a key role in the shoreline dynamics of this region.

Sand spits are common in wave-dominated environments; with enough sand supply, they can evolve to affect circulation and navigation in channels or inlets. The focus of this paper is on the navigation channel of the Sisal Port, located on the northwestern Yucatan Peninsula (YP) coast, where a sand spit grew and was monitored from its formation (June 2018) until navigation was practically blocked (November 2018). The YP coast is characterized as being microtidal, with significant wave heights ranging from 0.1 to 0.4 m (April to September), and in the presence of high energy events (cold fronts and storms), waves can reach heights of up to 2.5 m offshore at 10 m depth (October to February). Prior to the beginning of UAV surveys, we used photos (June–July 2018) from a stationary field camera and hydrodynamic data from models (WaveWatch III for waves and MARV software for tidal levels) to generate a qualitative description of the sand spit in the channel. Combining products from UAVs flights (DEMs) and hydrodynamic measurements (wave energy flux), we characterized the behavior and response of the sand spit, from its formation near the jetty head, through its consolidation in October 2018, to when a cold front with H_S ∼2.5 m breached it in mid-November. The results show that spit formation takes place during calm conditions (e.g., periods dominated by sea breezes), and depending on the energy threshold of high energetic events, this new spit will consolidate or be breached. Migration of the spit is related to overwash events and changes in wave direction. The presented methodology provides a well-rounded tool for characterizing the morphological behavior of spits on a shallow coast, which can be useful for improving coastal management.

Massive quantities of the pelagic brown macroalgae Sargassum spp. (sargassum) have been invading the Caribbean and West African shores since 2011, causing devastating effects on the coastal ecosystem and local economy. Little is known about sargassum beaching dynamics and the capacity of the coastal system to naturally remove beached sargassum. Here, we characterize the temporal variation in arriving and beached sargassum in a reef lagoon using a 5.2-year data set of hourly optical imagery, and identify the governing hydrometeorological conditions. Image classification reveals interannual variability in the start, duration, and intensity of the sargassum arrival season. Arrivals are associated with relatively low energy onshore directed winds and waves, and offshore abundance of sargassum. Furthermore, nearshore sargassum mat size is found to decrease with decreasing wave/wind energy. Once sargassum beaches, a berm of wrack is formed. Natural wrack removal was observed under elevated water levels and increased wave action. Three types of wrack removal were distinguished, depending on the water level (Formula presented.) with respect to the berm crest height (Formula presented.) and berm crest toe (Formula presented.) : gradual berm destruction with gaps developing in the seaward berm edge that grow larger with time (Type I; (Formula presented.)) and abrupt berm destruction with part of the wrack depositing on the upper beach (Type II; (Formula presented.)) or in the dunes (Type III; (Formula presented.)). Higher energy waves activate the reef circulation, which is suspected to flush part of the wrack out of the reef lagoon. We propose a conceptual model of nearshore sargassum dynamics in a reef lagoon system.